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Full text of "Text Book Of Mechanical Engineering"

Appendix II.

797

strength. It resists shock, is very uniform in structure, flanges
well, and is less corrodible than mild steel. It is therefore suit-
able for shafts, propellers, ship plates, boiler plates, large guns,
and lastly, when surface-hardened, for armour plates. Evidently
the function of the nickel is to prevent the shortness caused by
carbon, while permitting and even assisting the latter to exercise
its strength-giving property; it is, however, unable to confer hard-
ness without the assistance of the carbon, but increases that
hardening capacity.

The Harvey Process is applied to armour plate to give it
such extreme surface-hardness as will resist the attack of shell.
The process is essentially one of part cementation, or the intro-
duction of carbon to a given depth, and has been practised in
England by laying the plate on a shallow fireclay box filled with
charcoal, luting with fireclay, and keeping at 2400° Fahr. for
several weeks. As the plates weighed 30 or 40 tons each, the
risk of breaking the boxes was very great. This objection is
removed at the Bethlehem Steel Works, U.S.A., where two plates
are hardened simultaneously, face to face, but with 8 ins. of
•charcoal dust between. They are then placed on supports within
a furnace, thickly luted with sand and fireclay, and gradually
heated up to 1700° Fahr., remaining at that temperature for 8 or
10 days, after which they are taken out and laid on supports in
an empty tank, so as to keep them apart, and allow water pipes
to pass between and around them. From these pipes a spray of ice-
cold water is directed on the plates for about an hour, and the final
cooling is done in an oil tank. The oxide on the plate surface
is afterwards removed by a pneumatic chipping-chisel (p. 949).

Armour plates are thus hardened to a depth of about if ins.,
and cannot be drilled or otherwise machined unless locally
softened by an annealing process. For this purpose an electric
current of large volume, from an alternating dynamo, is sent
through the plate at the required place, heating it to 1000° Fahr.;
.and the temperature is then let down gradually to a dull red,
which is tested by the burning of a pine stick in contact with the
plate. The electrical principle involved is exactly the same as
that in the Thomson process of welding (p. 329), the dynamo
providing a current of TOO amperes at 300 volts, which is changed